There are several processes described in the prior art wherein coal is separated from the clay particles (ash) by froth flotation, oil agglomeration or sink/float sedimentation techniques. The oil agglomeration technique utilizes partitioning of the coal particles into an oil droplet or at least preferentially wetting of the coal by the oil to form an agglomerate. This does not generally allow for additional deashing of the coal-oil phase and generally the oil is left on the coal to be burned with the coal. Froth flotation techniques utilize the preferential wetting of the coal with a solvent and the preferential clinging of air bubbles to the hydrophobic surfaces. While some countercurrent repulping of the coal and reflotation can be conducted, usually in a countercurrent manner, froth flotation is usually not efficient enough to recover coal particles smaller than about 100 mesh.
In addition, there have been several disclosures in the prior art of the separation of the coal from the clay particles employing chlorinated hydrocarbon solvent/water solvent mixtures using liquid cyclones and/or rotary vacuum disc filters after partitioning for the separation of the two partitioned components to remove and recover the solvents.
None of these techniques are sufficiently efficient, by today's standards, to remove ash from the coal and enhance the coal burning Btu content on the pound basis nor to effectively bring the pyrite or sulfide sulfur components to a level low enough to effectively employ present day sulfurous gas removal from the combustion gases. It would, therefore, be advantageous to have a process which is capable of handling large quantities of coal and its attendant clay and ash in an economical manner.